Authentication-Results Registration for Differentiating among Cryptographic Results
RFC 6008

Internet Engineering Task Force (IETF)                      M. Kucherawy
Request for Comments: 6008                               Cloudmark, Inc.
Category: Standards Track                                 September 2010
ISSN: 2070-1721

        Authentication-Results Registration for Differentiating
                      among Cryptographic Results

Abstract

   This memo updates the registry of properties in Authentication-
   Results: message header fields to allow a multiple-result report to
   distinguish among one or more cryptographic signatures on a message,
   thus associating specific results with the signatures they represent.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6008.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Keywords  . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   4.  Definition  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
     6.1.  Improvement . . . . . . . . . . . . . . . . . . . . . . . . 4
     6.2.  Result Forgeries  . . . . . . . . . . . . . . . . . . . . . 4
     6.3.  New Schemes with Small Signatures . . . . . . . . . . . . . 4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 5
   Appendix A.  Authentication-Results Example . . . . . . . . . . . . 6
     A.1.  Multiple DKIM Signatures with One Failure . . . . . . . . . 6
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . . . 7

1.  Introduction

   [AUTHRES] defined a new header field for electronic mail messages
   that presents the results of a message authentication effort in a
   machine-readable format.  Absent from that specification was the
   means by which the results of two cryptographic signatures, such as
   those provided by [DKIM], can both have results reported in an
   unambiguous manner.

   Fortunately, [AUTHRES] created IANA registries of reporting
   properties, enabling an easy remedy for this problem.  This memo thus
   registers an additional reporting property allowing a result to be
   associated with a specific digital signature.

2.  Keywords

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

3.  Discussion

   A message can contain multiple signatures of a common sender
   authentication mechanism, such as [DKIM].  For example, a DomainKeys
   Identified Mail (DKIM) signer could apply signatures using two or
   more different message canonicalization algorithms to determine the
   resistance of each to being broken in transit.

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   By applying supported "ptype.property" combinations (cf. the ABNF in
   [AUTHRES]), a result can be associated with a given signature
   provided the signatures are all unique within one of the registered
   values (e.g., all of them had unique "header.d" or "header.i"
   values).  This is not guaranteed, however; a single signing agent
   might have practical reasons for affixing multiple signatures with
   the same "d=" values while varying other signature parameters.  This
   means one could get a "dkim=pass" and "dkim=fail" result
   simultaneously on verification, which is clearly ambiguous.

   It is thus necessary either to create or to identify a signature
   attribute guaranteed to be unique, such that it is possible to
   unambiguously associate a result with the signature to which it
   refers.

   Collisions during general use of SHA1 and SHA256 are uncommon (see
   [HASH-ATTACKS]), and RSA key signing mechanisms are resilient to
   producing common substrings.  Thus, the actual digital signature for
   a cryptographic signing of the message is an ideal property for such
   a unique identification.  It is not, however, necessary to include
   the entire digital signature in an [AUTHRES] header field just to
   identify which result goes with which signature; since the signatures
   will almost always be substantially different, it is anticipated that
   only the first several bytes of a signature will be needed for
   disambiguating results.

4.  Definition

   This memo adds the "header.b" reporting item to the IANA "Email
   Authentication Methods" registry created upon publication of
   [AUTHRES].  The value associated with this item in the header field
   MUST be at least the first eight characters of the digital signature
   (the "b=" tag from a DKIM-Signature) for which a result is being
   relayed, and MUST be long enough to be unique among the results being
   reported.  Where the total length of the digital signature is fewer
   than eight characters, the entire signature MUST be included.
   Matching of the value of this item against the signature itself MUST
   be case-sensitive.

   If an evaluating agent observes that, despite the use of this
   disambiguating tag, unequal authentication results are offered about
   the same signature from the same trusted authserv-id, that agent
   SHOULD ignore all such results.

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5.  IANA Considerations

   Per [IANA-CONSID], the following item is added to the "Email
   Authentication Methods" registry:

   +------------+----------+--------+----------------+-----------------+
   |   Method   | Defined  | ptype  | property       | value           |
   +------------+----------+--------+----------------+-----------------+
   |    dkim    | RFC4871  | header | b              | full or partial |
   |            |          |        |                | value of        |
   |            |          |        |                | signature "b"   |
   |            |          |        |                | tag             |
   +------------+----------+--------+----------------+-----------------+

6.  Security Considerations

   [AUTHRES] discussed general security considerations regarding the use
   of this header field.  The following new security considerations
   apply when adding or processing this new ptype/property combination:

6.1.  Improvement

   Rather than introducing a new security issue, this can be seen to fix
   a security weakness of the original specification: Useful information
   can now be obtained from results that could previously have been
   ambiguous and thus obscured or, worse, misinterpreted.

6.2.  Result Forgeries

   An attacker could copy a valid signature and add it to a message in
   transit, modifying some portion of it.  This could cause two results
   to be provided for the same "header.b" value even if the entire "b="
   string is used in an attempt to differentiate the results.  This
   attack could cause an ambiguous result to be relayed and possibly
   neutralize any benefit given to a "pass" result that would have
   otherwise occurred, possibly impacting the delivery of valid
   messages.

   It is worth noting, however, that a false negative ("fail") can be
   generated in this way, but it is extremely difficult to create a
   false positive ("pass") through such an attack.  Thus, a cautious
   implementation could discard the false negative in that instance.

6.3.  New Schemes with Small Signatures

   Should a new signing scheme be introduced with a signature whose
   length is less than eight characters, Section 4 specifies that the
   entire signature must be used.  The obvious concern in such a case

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   would be that the signature scheme is itself prone to collisions,
   making the value reported by this field not useful.  In such cases,
   the risk is created by the likelihood of collisions and not by this
   mechanism; furthermore, Section 4 recommends the results be ignored
   if that were to occur, preventing the application of an ambiguous
   result.

7.  References

7.1.  Normative References

   [AUTHRES]       Kucherawy, M., "Message Header Field for Indicating
                   Message Authentication Status", RFC 5451, April 2009.

   [DKIM]          Allman, E., Callas, J., Delany, M., Libbey, M.,
                   Fenton, J., and M. Thomas, "DomainKeys Identified
                   Mail (DKIM) Signatures", RFC 4871, May 2007.

   [KEYWORDS]      Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2.  Informative References

   [HASH-ATTACKS]  Hoffman, P. and B. Schneier, "Attacks on
                   Cryptographic Hashes in Internet Protocols",
                   RFC 4270, November 2005.

   [IANA-CONSID]   Narten, T. and H. Alvestrand, "Guidelines for Writing
                   an IANA Considerations Section in RFCs", BCP 26,
                   RFC 5226, May 2008.

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Appendix A.  Authentication-Results Example

   This section presents an example of the use of this new item header
   field to indicate unambiguous authentication results.

A.1.  Multiple DKIM Signatures with One Failure

   A message that had two DKIM signatures applied by the same domain,
   one of which failed:

        Authentication-Results: mail-router.example.net;
              dkim=pass (good signature) header.d=newyork.example.com
                    header.b=oINEO8hg;
              dkim=fail (bad signature) header.d=newyork.example.com
                    header.b=EToRSuvU
        Received: from newyork.example.com
                  (newyork.example.com [192.0.2.250])
              by mail-router.example.net (8.11.6/8.11.6)
                  for <recipient@example.net>
                  with ESMTP id i7PK0sH7021929;
              Fri, Feb 15 2002 17:19:22 -0800
        DKIM-Signature: v=1; a=rsa-sha256; s=rashani;
              d=newyork.example.com;
              t=1188964191; c=relaxed/simple;
              h=From:Date:To:Message-Id:Subject;
              bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
              b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3=
        DKIM-Signature: v=1; a=rsa-sha256; s=rashani;
              d=newyork.example.com;
              t=1188964191; c=simple/simple;
              h=From:Date:To:Message-Id:Subject;
              bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
              b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
        From: sender@newyork.example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: meetings@example.net
        Message-Id: <12345.abc@newyork.example.com>
        Subject: here's a sample

   Example 1: Header field reporting results from multiple signatures
   added at initial signing

   Here we see an example of a message that was signed twice by the
   author's ADministrative Management Domain (ADMD).  One signature used
   "relaxed" header canonicalization, and the other used "simple" header
   canonicalization; both used "simple" body canonicalization.

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   Presumably due to a change in one of the five header fields covered
   by the two signatures, the former signature passed, while the latter
   signature failed to verify.  In particular, the "relaxed" header
   canonicalization of [DKIM] is resilient to changes in whitespace in
   the header, while "simple" is not, and the latter is the one that
   failed in this example.

   The item registered by this memo allows an evaluation module to
   determine which DKIM result goes with which signature.  Without the
   "header.b" portion of the result, it is unclear which one passed and
   which one failed.

Appendix B.  Acknowledgements

   The author wishes to acknowledge the following for their review and
   constructive criticism of this proposal: Dave Crocker, Tony Hansen,
   Eliot Lear, S. Moonesamy, and Alessandro Vesely.

Author's Address

   Murray S. Kucherawy
   Cloudmark, Inc.
   128 King St., 2nd Floor
   San Francisco, CA  94107
   US

   Phone: +1 415 946 3800
   EMail: msk@cloudmark.com

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